The defender was a free agent after his contract with AS Roma expired.
“I keep all the Roma teammates in my heart, as I’ll never forget the city and its fans, who did so much for me. Now I am at Milan and very proud to be part of this club,” he said.
“I had seven marvellous years at Roma and it’s partly thanks to them that I grew as a man and a player to earn this opportunity with Milan.
“It was my choice, as I could’ve joined Real Madrid this summer. In the past I chose Roma ahead of Manchester United. I dreamed of a club like this since childhood and it’s an important shop window, so I can hope to earn a place in the France squad for Euro 2012.”